X-ray tube anticathodes are rotating disks constituted by a support coated at least partly with an active film made of a refractory metal. They are used in medical instruments, such as scanners.
The current trend of medical instrument manufacturers is to be able to increase the power received by the anticathode and/or to reduce the size of the impact spot of the electron bombardment in such a way as to improve definition of the image obtained. This desire to increase the power or reduce the size of the spot is limited by the slow progress of the anticathode in evacuating the stored heat and consequently by the temperature of the focal track rising to the temperature for melting the material constituting the active film of the anticathode on which this track is formed.
Most frequently, the support of the anticathode is made of a material containing carbon and constituted by a polycrystalline graphite whose coefficient of expansion is compatible with that of the refractory metal, such as tungsten, a tungsten/rhenium alloy or a molybdene-based alloy which is secured (for example, by brazing) or deposited (for example, in a vapor phase or by electrolytic means) onto this support.
So as to keep the temperature of the focal track to acceptable values in a steady or transient state whilst increasing the power or by reducing the size of the spot, one solution would consist of significantly increasing the rotation speed of the anticathodes so as to reach speeds equal to or greater than 20,000 rpm, for example, unfortunately, the polycrystalline graphites normally constituting the support of anticathodes do not possess sufficient mechanical resistance. In fact, they splinter under the effect of centrifugal force before reaching such a speed.
Furthermore, in conventional anticathodes with a graphite support coated with a rhenium/tungsten alloy film, it is essential to insert an extremely fine rhenium sub-film. In fact, from several hundreds of degrees, the carbon atoms of the graphite tend to migrate so as to form with the tungsten a fragile film of tungsten carbide provoking a loss of cohesion between the substrate and the active film and disturbing the thermal transfer. Up to a temperature of about 1200.degree. C., the rhenium prevents this migration and thus plays the role of an anticarbonizing barrier. However, beyond this temperature, the rhenium is increasingly less effective and the anticathode then exceeds its functioning limit. Moreover, the rhenium is an expensive substance and thus increases the cost of the anticathode.
Other less costly materials, such as SiC and TaC, may play the role of an anticarbonizing barrier, but the fact that an additional stage needs to be added to the method results in increasing the overall cost of said method.
The document EP-A-0 236 24A proposes a method to embody an anticathode from a composite support formed of carbon fibers embedded in a carbon matrix ("carbon/carbon" composite). Such a composite material possesses mechanical resistance much greater than the polycrystalline graphites used previously, which makes it possible to rotate the anticathode at an extremely high speed without the disk risking being splintered under the effect of centrifugal force.
Unfortunately, such a composite carbon/carbon material has a coefficient of expansion widely differing from that of the metallic film. Thus, the coefficient of expansion of a carbon/carbon composite is 0.5.10.sup.-6 .degree.K..sup.-1 at 25.degree. C. and 2.10.sup.6 .degree.K..sup.-1 at 1000.degree. C., whereas the coefficient of expansion of a rhenium/tungsten alloy metal film is 4.10.sup.-6 .degree.K..sup.-1 at 25.degree. C. and 4.5.10.sup.-6 .degree.K..sup.-1 at 1000.degree. C.
To overcome this drawback, the document EP-A-0 236 241 proposes depositing the metallic film on a graphite substrate with a coefficient of expansion similar to that of the metal, this graphite substrate being associated by any means (brazing, glueing, embedding, etc) with the carbon/carbon composite support.
Thus, an anticathode is embodied having a mechanic resistance enabling it to rotate at a high speed, but the production of the anticathode is complicated by virtue of the incompatibility of firstly the coefficients of expansion of the carbon/carbon composite support and secondly of the metal film/graphite assembly. In addition, the presence of a graphite substrate between the metal film and composite support requires the insertion between the graphite substrate and the metal film an extremely fine sub-coating of rhenium to be used as an anticarbonizing barrier, as in conventional anodes with a graphite support. The use temperature of the anticathode is thus limited and increases its cost.